This study numerically models energy production in hydrothermal reservoirs using both CO \(_{\varvec{2}}\) and water as working fluids. Unlike past studies, this work considers multi-phase flow to model the effect of water displacement by supercritical CO \(_{\varvec{2}}\) (ScCO \(_{\varvec{2}}\) ). Several simulations are performed to systematically evaluate the influence of reservoir and injection temperatures as well as fracture networks on energy production rates. Furthermore, the efficacy of water and CO \(_{\varvec{2}}\) as working fluids is compared by evaluating their respective energy production rates in moderate- to high-enthalpy reservoirs. For identical mass injection rates, energy production rates at early stages are higher in reservoirs with ScCO \(_{\varvec{2}}\) than in those with water, due to the higher thermal expansivity of ScCO \(_{\varvec{2}}\) . By contrast, at later stages, energy production rates in CO \(_{\varvec{2}}\) -based reservoirs are lower than in water-based reservoirs due to the lower heat capacity of ScCO \(_{\varvec{2}}\) , although this difference can be minimized by carefully calibrating the injection temperature. Additionally, the injection pressure required for CO \(_{\varvec{2}}\) -based reservoirs is significantly lower than for water-based reservoirs, because ScCO \(_{\varvec{2}}\) is less viscous than water. The findings from this study provide useful insights into the use of CO \(_{\varvec{2}}\) as a working fluid in enhanced geothermal systems.
